The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Current and future wireless communication systems focus on higher capacity. Two main methods of capacity increase in modern cellular systems are to provide Multiple Input, Multiple Output (“MIMO”) signals and to provide cell splitting capabilities.
MIMO is a method used to increase the capacity of radio links by using multiple (or multi-port) antennas. However, MIMO antennas produce signals that can interfere with one another without high isolation between antenna ports (See, “MIMO Processing for 4G and Beyond: Fundamentals and Evolution” by Mario Marques da Silva, Francisco A. Monteiro, CRC Press, 2014). In the MIMO antenna disclosed in U.S. Pat. No. 8,482,478 to Hartenstein titled “MIMO Antenna System,” absorber material is used to improve isolation between antenna ports. However, Hartenstein's absorber causes signal degradation and passive inter-modulation (PIM) issues, which is not acceptable for 3G, 4G/LTE cellular systems. Other MIMO antennas, such as those disclosed in US 2011/0135308 to Tarlazzi et. al., are quite bulky and are not multi-beam.
One of the more effective ways to provide cell splitting capabilities is by use of multi-beam antennas (e.g. U.S. publication 2016/0013563 to Timofeev titled “Wideband Twin Beam Antenna Array,” U.S. publication 2011/0205119 to Timofeev et al. titled “Dual-Beam Sector Antenna and Array”, U.S. Pat. No. 6,081,233 to Johannisson titled “Butler Beam Port Combining For Hexagonal Cell Coverage,” and U.S. Pat. No. 7,605,768 to Ebling et al. titled “Multi-beam Antenna”). However, existing multi-beam antennas cannot be used effectively with MIMO systems because of poor isolation between antenna ports. Antenna ports in other attempted solutions typically exhibit 13-15 dB isolation, which is not enough for a wide operational frequency band.
Therefore, existing multi-beam antennas attempted by others cannot provide wideband MIMO performance required for current and future cellular communications, and existing MIMO antennas are not suitable for multi-beam operation.
All publications identified herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Thus, there is still a need for improved antennas can increase capacity in wireless communication systems.